1. Field of the Invention
The invention relates to a method and apparatus for reducing stress during stent manufacture. Particularly, the invention is directed to a method and apparatus for expanding a stent using primarily radial loads thereby reducing the stresses that are generated due to the axial loads applied during loading of the stent onto a mandrel or other expansion device. The method of the invention provides for the stepwise expansion of shape memory stents, while reducing the overall stresses that the stent encounters, and thereby improving manufacturing yields due to fractured struts during expansion.
2. Description of Related Art
Cardiovascular disease is prevalent in the United States and in other parts of the world. One manifestation of cardiovascular disease is atherosclerosis, which is the buildup of plaque (or fatty deposits) on the walls of blood vessels, such as coronary arteries. This buildup of plaque can grow large enough to reduce blood flow through the blood vessel. Serious damage results when an area of plaque ruptures and forms a clot, which travels to another part of the body. If the blood vessels that feed the heart are blocked, a heart attack results. If the blood vessels to the brain are blocked, a stroke results. Thus, atherosclerosis can be fatal for some people.
Typically, physicians treat atherosclerosis by implanting a tubular endoprosthesis such as a stent at the narrowed or blocked segment of the blood vessel, which widens and holds open the blood vessel. To perform this procedure the stent is delivered to the site of the lesion in the blood vessel by a catheter assembly, otherwise known as a stent delivery device. The stent delivery device enters the vasculature of the patient through the femoral artery and travels through a tortuous path to the site of the lesion. The physician positions the stent across the lesion and deploys the stent so that the stent forces the plaque against the inside wall of the blood vessel (or lumen) and maintains its expanded configuration so that the patency of the blood vessel is maintained.
The term “stent” has come into widespread use to denote any of a large variety of spring-like support structures, in the form of a tube which is open at both ends, which can be implanted inside a blood vessel or other tubular body conduit, to help keep the vessel or conduit open. Stents may be used following balloon angioplasty to prevent restenosis and may, more generally, be used in repairing any of a number of tubular body conduits, such as those in the vascular, biliary, genitourinary, gastrointestinal and respiratory systems, among others, which have narrowed, weakened, distorted, distended or otherwise deformed, typically as a result of any of a number of pathological conditions.
Typically, the stent is delivered inside the body by a catheter that supports the stent in a compacted form as it is transported to the desired site. Upon reaching the site, the stent is expanded so that it engages the walls of the lumen. The expansion mechanism may involve forcing the stent to expand radially outward, for example, by inflation of a balloon carried by the catheter, to inelastically deform the stent and fix it at a predetermined expanded position in contact with the lumen wall. The expansion balloon can then be deflated and the catheter removed.
In another technique, the stent is formed of a highly elastic material that will self-expand after being compacted. During introduction into the body, the stent is restrained in the compacted condition. When the stent has been delivered to the desired site for implantation, the restraint is removed, allowing the stent to self-expand by its own internal elastic restoring force.
An effective stent must possess a number of important and very specific characteristics. Specifically, the stent should be chemically and biologically inert to its surroundings and should not react with, or otherwise stimulate, the living tissues around it. The stent must further be such that it will stay in the correct position and continue to support the tubular body conduit into which it is implanted over extended periods of time. Further, the stent must have the ability to return to its prescribed in-place diameter after the stent diameter has been significantly reduced prior to its insertion, usually tightly wrapped on a catheter, into the tubular body conduit. An example of such a stent in the prior art is disclosed in U.S. Pat. No. 5,827,321, the entirety of which is hereby incorporated by reference.
A variety of methods and systems are known for manufacturing stents, and for imparting a desired geometry onto the stent structure. Conventional methods of manufacturing stents required the expansion of the stents from a smaller diameter, or “as cut” position, to a larger diameter corresponding to the stent configuration as deployed in the patient. This expansion is typically performed by the intricate process of providing an initial heat treatment stage followed by the forcible sliding of the stents over a mandrel, and providing a subsequent heat treatment stage.
Such conventional methods and systems generally have been considered satisfactory for their intended purpose. Recently, however, there is a need to reduce or eliminate the stress induced on the stent during application of the axial force required to forcibly slide the stent over the mandrel. The stresses generated within the stent material as the stent encounters radial loads and axial loads while being placed onto the mandrels can result in localized deformities such as strut fracture, kink, and flare. The presence of such deformities can jeopardize the structural integrity and performance characteristics of the stent. Further, such deformities can damage tissue in the lumen wall of the patient. Consequently, the conventional methods for expanding stents requires extensive quality control and results in low product yield.
Additionally, the prior art method of expanding stents is disadvantageous in that the process must be performed in various discrete stages requiring numerous mandrels of differing sizes to provide incremental expansion in order to avoid damaging the stent. In many instances the requisite tooling and discrete process steps will reach a level that is too burdensome and complex to be performed in a cost effective manner. Examples of such prior art expansion techniques are disclosed in U.S. Pat. No. 6,305,436 and U.S. Pat. No. 6,402,779, each of which is hereby incorporated by reference in their entirety.
As evident from the related art, conventional methods often provide inadequate stent expansion techniques and cost prohibitive systems.
There thus remains a need for an efficient and economic method and system to provide for the stepwise expansion of shape memory stents, while reducing the overall stresses that the stent encounters, and thereby improving manufacturing yields due to fractured struts during expansion.